To obtain uniform heating in a microwave oven, it is often desirable to move the food item about within the oven to expose it to regions of different microwave intensity. A well known problem with common household microwave ovens is the nonhomogeneous energy distribution resulting from a complex standing wave pattern that sets up when microwave energy is propagated into the microwave oven cavity. A useful analogy is a light beam entering a mirrored room. The beam reflects and scatters from objects in the room and from the walls of the room and in the process crosses its own path many times. In the case of radiant microwave energy, the wavelength is such that where the waves intersect heating is increased or decreased depending on the relative phases of the respective waves at the points of intersection. Where the phases oppose and waves cancel, heating is decreased and a cold spot results. Where waves reinforce, heating is increased and a hot spot results. Hot and cold spots occur only where absorbing materials convert the different microwave energy densities to corresponding various heating levels. The location of the hot and cold spots depends on the geometry of the oven and the type and position of energy absorbing or reflecting objects placed in the oven.
In attempts to solve the problem, manufacturers of frozen food preparations often include directions recommending that the user perform the inconvenient task of repositioning a food container from time to time during the microwave cooking period. Ovens often include a "mode stirrer" for reflecting and distributing microwave energy more uniformly inside the oven. Various turntables and rotisseries have been developed to rotate the food itself during cooking. Non-removable turn tables and rotisseries have been developed and included during initial manufacture of more expensive oven models. Such rotisseries are generally the least reliable component and add considerably to the cost of a microwave oven. Accordingly, many oven units do not include a means for rotating the food.
Various turntables have been developed for retro-fitting a microwave oven with a removable rotisserie means. Examples of such prior art turntables appear in the following patents:
Beh et al. U.S. Pat. No. 4,254,319 discloses a portable turntable for a microwave oven wherein microwave energy heats a reservoir of water to generate steam that powers a turbine geared to the turntable.
Bowen et al. U.S. Pat. No. 4,434,343 discloses a turn table for a microwave oven which is powered by a wind-up clock motor arrangement.
Park U.S. Pat. No. 4,725,703 discloses a microwave turntable with a cam and gear arrangement for lifting and turning a food item in a microwave oven.
Prior art turntables suffer from complexity and fail to provide in a single unit all the advantages of the present invention. There is a continuing need for a simple, low cost rotisserie that more effectively distributes microwave energy in an absorbing item.